Surface characterisation of V2O5/TiO2 catalytic system

Samples of the V2O5/TiO2 system were prepared by the sol-gel method and cal cined at different temperatures. Surface species of vanadium, their dispers ion, as well as the structural evolution of the system were analysed by XRD , Raman, EPR, and XPS techniques. The results of XRD showed the evolution o f TiO2 from anatase phase to rutile. phase. The Raman spectra for calcinati on temperatures up to 500 degreesC showed a good dispersion of vanadium ove r titania in the form of monomeric vanadyl groups (V4+) and polymeric vanad ates (V5+). At least three families of V4+ ions were identified by EPR inve stigations. Two kinds of isolated V4+ species are placed in sites of octahe dral symmetry, substituting Ti4+ in the rutile phase. The third is formed b y pairs of V4+ species on the surface of titania. Above 500 degreesC part o f superficial V4+ is inserted into the,matrix of titania and part is oxidiz ed to V5+. The XPS results showed that the V/Ti ratio rises with increasing calcination temperature, indicating a smaller dispersion of vanadium.